Larry Steckel, Ph.D., associate professor, plant science, University of Tennessee
Larry Steckel, Ph.D., associate professor, plant science, University of Tennessee

Palmer pigweed is killing off soybean fields right and left in the Midsouth, according to Larry Steckel, Ph.D., associate professor, plant science, University of Tennessee.

“We lost more soybean fields in 2011 to glyphosate-resistant Palmer pigweed than all the previous years combined,” Steckel said. The word lost means unharvestable in the professor’s terminology.

“We are having a pigweed heart attack right now,” Steckel said. He used the analogy of a person having to deal with health concerns. He explained that the doctor tells someone they need to change their lifestyle or they can expect problems with their heart, but too many people don’t change until they have a heart attack and are then forced to make changes.

“Nobody wants to change until it happens to them,” but so many growers have had glyphosate-resistant Palmer pigweed (also called Palmer amaranth) destroy their yields that they are changing. They’ve had the Palmer pigweed heart attack.

“Hopefully, we have kind of learned from our past ways, and we are going to start employing systems instead of relying on one herbicide. It is not going to be painless. The days of going out with 90-foot booms, spraying at 18 miles per hour with the same weed control on every acre is over,” he said.

Steckel warned that it isn’t just Palmer pigweed that can cause this type of problem and that Midwest farmers need to be proactive against the aggressive weeds showing up as glyphosate resistant. He said farmers who haven’t been attacked by glyphosate-resistant weeds need to be proactive.

Weed control has to be much more intense than the easy, simple use of only glyphosate herbicides. More than one mode of action pre-emergence herbicides with residual activity has to be applied with the expectation of one or two post-emerge applications with additional modes of action being necessary.

Of the nine weeds verified as resistant to glyphosate found in Tennessee during the last 10 years, Palmer pigweed is probably the most difficult with which to contend. As an aside, Steckel said goosegrass and bluegrass were added to the list of resistant vegetation found in the state during 2011.

Palmer pigweed is definitely a hearty plant that loves hot weather much more than the crops grown in the Midsouth. Steckel explained that the weed is native to the desert Southwest. It will grow two to three inches per day above ground even in 100-degree temperatures, and it will put down a tap root five feet long. He documented that the weed grew five feet tall in 20 days, or three inches per day, in at least one field during 2011.

The current control measures in soybeans typically include using post-emerge PPO mode-of-action herbicides such as Flexstar, Cobra and Blazer, but these need to be applied when the weed is two inches tall. Control of two-inch resistant Palmer pigweed usually results in 95 percent control, but if the weed is allowed to reach four inches before the herbicide application then the result is only about 75 percent control. Naturally, control continues to decrease drastically every 24 hours that the pigweed is emerged.

“With only 75 percent control, you won’t combine that field of soybeans,” Steckel said.

He wasn’t extremely positive about relying on post-emergence herbicide applications for Palmer pigweed or other glyphosate-resistant weeds because timing is so critical and weather will interfere in application almost every year. He is highly doubtful that there is enough application equipment available to do the number of applications exactly when the applications need done.

A warning was to not try and spray more acres per day late into the evening or quite early in the morning, even though it is possible with today’s precision equipment allowing spraying in the dark. Temperature inversions commonly occur in the Midwest and South, which can cause major problems with off-target crop injury.

Steckel is telling farmers across the nation to learn from the Midsouth growers and do a better job in herbicide stewardship. “If you are filling out a scorecard, we are failing when it comes to trying to steward glyphosate.”

Steckel made at least two presentations during the Commodity Classic in Nashville as he was supportive of messages by Bayer CropScience and BASF concerning herbicide-resistant weed control.